WCA50 Telephone assessment

Hi, I posted my WCA50(first one ever) in November last year and have just received a letter informing me that a telephone assessment has been set up for later this month. I included lots of evidence from consultants/professionals confirming my diagnosis, The main ones are a worn-out hip that requires replacing which severely restricts my mobility to well under the 50-meter mark, but surgery cannot be undertaken at the moment due to my chronic anemia. Several letters of support by my gp and other professionals where sent off with wca50 form confirming everything that I wrote in the form I explained at length on the form how all my disabilities affect me and how my chronic anemia is causing me to faint regularly I included copies of my latest blood tests showing how my hemoglobin estimation was 51 g/l when it should be 120-160 g/l it is only up to 65 g/l after months of medication.
So I am wondering what other information they require from me that cannot be obtained by looking at the WCA50 or reading the letters from various professionals confirming all of my diagnosis/disabilities.
Do they think myself and my GP and the other professionals are all telling a pack of lies?
Or could it be that they get paid more money from the DWP by carrying out telephone assessments compared to if they sent a form back to DWP saying this person is clearly unfit for work?
Any opinions or other members' experiences of WCA50 forms and telephone assessments would be really appreciated as I am totally new to all this.
Thanks
Shafilia